A problem which is of considerable concern to electric utilities is posed by cyclic loading of large steam power plants. Steam turbines in the 200 MW class and larger, typically with steam conditions of 1000.degree. F. and 2400 psi or above, have strict restraints on the rate at which steam temperatures can be varied. Whenever a steam temperature change is imposed, the large mass of metal in the turbine casing and rotor take time to reach a new equilibrium. In the transient condition, thermal stresses are induced that are capable of causing permanent damage. The conventional gas, oil or coal-fired boiler, and particularly the pulverized coal-fired boiler, provides a constant steam temperature over a very limited load range, typically above about two thirds of its rated capacity.
During low load running or start up, steam temperatures may be more than 300.degree. F. below the design level, necessitating extended periods for cooling the turbine before shut down or load reduction, and for reheating the turbine before reloading. This is costly in terms of reduced efficiency, steam dumping and possible thermal cycling damage.
The present invention provides a novel approach to the design of a steam boiler in which the final steam temperature may be matched to the turbine over the whole load range, including hot and warm starts.